[Phase 1] Extract config_gen modules for Manager, Storage Proxy, Webserver

## Summary

Extract Manager configuration generation (manager.toml, alembic.ini, etcd config, fixtures) into a shared module.

## Current Duplication

- `context.py:configure_manager()` — sed + tomlkit modifications
- `deploy/cores/manager/` — Jinja2 templates
- Redis sentinel/single mode logic duplicated
- Fixture generation logic duplicated

## Complexity: HIGH

Manager has complex config with Redis mode selection (sentinel vs single), etcd config, and fixture generation with user/group data.

## Approach

Create `src/ai/backend/install/config_gen/manager.py` with config dict generation functions.

JIRA Issue: BA-5566

Start by reading context.py:configure_manager() and the templates under deploy/cores/manager/. Trace the duplicated Redis mode, etcd, and fixture-generation paths, then define the scope of src/ai_backend/install/config_gen/manager.py. Done means Manager configuration and fixture generation use the shared module without the current duplicated logic.

Written by the indexing model from the issue text.